Hello, @Antonis,
We have recently tested your website on GTMetrix and the results can be found here: https://gtmetrix.com/reports/demo01.coreit.gr/6vMrlJu5/. The top issues identified by GTMetrix are listed in the report.
Reduce initial server response time (Keep the server response time for the main document short because all other requests depend on it. Consider using a plugin like WP Rocket to significantly reduce server response time » https://gtmetrix.com/go/wprocket-general)
Avoid chaining critical requests (The Critical Request Chains show you what resources are loaded with a high priority. Consider reducing the length of chains, reducing the download size of resources, or deferring the download of unnecessary resources to improve page load) https://postimg.cc/CZ7FY8g5
Avoid an excessive DOM size (A large DOM will increase memory usage, cause longer style calculations, and produce costly layout reflows. https://gtmetrix.com/avoid-an-excessive-dom-size.html)
Use a Content Delivery Network (CDN) CDNs can give an equally fast web experience to your users across the globe.
Further, you can read our theme documentation on speed here: https://xstore.helpscoutdocs.com/article/100-how-to-speed-up-my-website & https://xstore.helpscoutdocs.com/article/128-how-to-improve-wp-rocket-settings-with-xstore
Kind Regards,
8Themes Team.